1. Field of the Invention
This invention generally relates to a photo detector, and more particularly, to a photo detector being used to detect the narrow bandwidth light energy.
2. Description of the Prior Art
With the rapid development of the human life style and the industries, fossil fuels are extensively used which results in the serious problem of air pollution. Hence, detecting the environmental gas has attracted more and more attentions in recent years. Due to the different molecular vibration spectra, a gas usually has strong absorption at one or more narrow bandwidth infrared wavelengths. The infrared detection system was the conventional optical method to detect the gas which consists of a broad band infrared light source, a detection chamber and a broad band photo detector. The photo detector with the narrow bandwidth absorption is expected to improve the detection sensitivity and decrease the system size.
At present, the infrared detector with narrow bandwidth absorption can be roughly divided into three types. The first type uses the semiconductor epitaxy technique to manufacture III-V quantum-well, quantum dot or quantum ring structure, and to cooperate the back-end process to perform the infrared detection element, and the infrared detector manufactured by the method adjust the structure parameters and the element proportion so as to adjust the detection wavelength. However, the infrared detector must be operated in low temperature environment and the process equipment is expensive.
The second type uses an infrared detector with full band (for example, the thermal pile) and sets an optical filter in front of the infrared detector for achieving to detect infrared light with narrow bandwidth. Although the infrared detector of the second type can operate in the room temperature environment, the use of the optical filter increases the system size and the manufacture cost.
The third type uses the infrared thermal imager manufactured by the semiconductor process and the micro electro mechanical system (MEMS), and the infrared thermal imager is also a resistive optical detector comprising a substrate and an amorphous silicon film. Specifically, the infrared absorption material is applied on the top of amorphous silicon film, the bottom of the amorphous silicon film is hollowed, and two sides thereof respectively mount a cantilever connected to the substrate that is used to the mechanical support and the electrical measure point. When the resistive optical detector receives the infrared light, the temperature of amorphous silicon film is changed, so as to change the electrical conductivity of the amorphous silicon film, and therefore, the resistance of amorphous silicon film is measured to achieve the infrared detection. Although this kind of the infrared detector is operated in the room temperature environment, it completely absorbs any light waveband without the limitation condition, and therefore, the kind of the infrared detector need to incorporate an optical filter to filter the undesired light waveband. As a result, the system size and the manufacturing cost are increased, and the use of cantilever structure reduces the effective area of the element so as to affect the operation efficiency.
For the reason that the conventional photo detector could not avoid to increase the system size in order to detect the narrow bandwidth light, a novel scheme is proposed that may adaptively avoid increasing the system size and detecting the narrow bandwidth.